The House Intelligence Committee several days ago began investigating the matter, but the apparent lack of impartiality by its chairman, Republican Devin Nunes, has called that probe into question.

Nunes revealed last week that he had received information confirming that the communications of President Donald Trump may have been the object of "incidental" monitoring by intelligence agencies during the final months of the Barack Obama administration, but he refused to reveal his sources or to detail what occurred to his committee colleagues.

According to The New York Times on Thursday, the secret sources of Nunes were two officials with Trump's current White House team, raising even further the doubts about the credibility of the information.

Trump claimed early in March that Obama ordered telephone calls at New York's Trump Tower intercepted, but both the FBI and the National Security Agency have denied having any evidence supporting that accusation.

However, amid the process of interviews to determine what happened, Nunes insinuated that the president was monitored, although not directly, thus supporting -- to a certain degree -- Trump's claim, although without any proof.

The efficacy of the House committee, which to date had led the congressional investigations into Russian election meddling, has thus been thrown into doubt, whereupon the senators decided to intensify their own probe.

Although neither of the two lawmakers wanted to comment on the questionable conduct of Nunes, they made clear that their investigation will be completely independent.

If the lawmakers politicise the process, their efforts will probably fail, said Burr, one of the first lawmakers to support Trump during the campaign, regarding the need for a bipartisan and transparent investigation.

The Republican acknowledged that his committee's big challenge is to clarify for the public whether Trump's campaign colluded with the Russian government to interfere in the election, given that the Kremlin's efforts to influence the election have been confirmed by US intelligence agencies.

Warner said Thursday that he had been informed that Moscow hired about 1,000 hackers to spread fake news on the social networks about then-Democratic presidential nominee Hillary Clinton, whom Trump defeated in the November vote.